1. Field of the Invention
This invention concerns underwater drilling of oil and gas wells. Specifically, it pertains to methods and apparatus used in underwater extended casing operations.
2. Description of the Prior Art
Increased activity in offshore drilling has resulted in a continuous search for better methods and apparatus in this area. To cope with the unique problems associated with underwater drilling various extended casing methods have been developed. Basically, extended casing methods have a well conductor anchored to the sea floor which provides support for a special underwater wellhead. The wellhead, in turn, supports a multiple number of casing strings and their respective casing hangers. The drilling platform is thus relieved of much of the structural support responsibilities of other methods. After drilling is completed, the well may be permanently abandoned, temporarily abandoned or immediately completed. For any of these options, the completion equipment may be installed at the sea floor, leaving the drilling platform free for relocation and freeing the underwater wellhead from the hazards of ocean going traffic and structural support problems. One such extended casing method is fully described in copending U.S. pat. application Ser. No. 572,599.
In the extended casing methods of the prior art, one or more intermediate casing strings, in addition to the conductor and the innermost production casing string, are usually supported in the wellhead. Casing extensions or risers are attached to these strings as they are lowered into place and landed. The extensions are connected at the surface to a blowout preventer for pressure control and also serve as a return for cement circulation. In the past it has been necessary to remove all casing extensions, except possibly the outer conductor riser, for installation of the tubing head with the collet connector flange for making connection with the underwater tree, the tubing hanger and tubing strings. This requires removal of the surface blowout prevention equipment. In some cases, for safety precautions, a bridge plug is set in the production casing prior to removal of the production casing riser. The tubing head is attached to the production casing hanger head and a high pressure riser extended back to the surface for reattachment of the preventer equipment. The bridge plug is then drilled out or otherwise and the well is then ready to receive tubing. These operations require additional equipment time, and consequently expenses.
Some methods have utilized underwater blowout preventers installed near the underwater wellhead. However, such preventers are very expensive and more complex to operate than the conventional above water type.